The Program Project Application is concerned with basic phenomena of conception and early pregnancy in mammals, including man. The general objectives are: (1) To study in depth the stages of conception and early pregnancy in mammals and man whose failure has been identified as possibly leading to infertility, or whose vulnerability may lead to new methods of controlling fertility. Since it is virtually impossible to study in detail these phases of human reproduction, we propose to study mechanisms which are specific to the reproduction processes of mammals, and pursue their regulation in depth. Where possible, critical experiments will be done on human material. (2) The topics to be investigated include some aspects of spermatogenesis, oogenesis, fertilization, cleavage and implantation. Emphasis is placed on the kinetic aspects of early development in the mammal and the interactions of the preimplantation stages with the local environment where the preimplantation embryonic stages develop within the female genital tract.